It is common practice in the manufacture of electronic assemblies to provide a printed circuit board as a base or carrier for circuit elements such as microprocessor chips, power die transistors, semiconductors and other circuit components. The printed circuit board may comprise a sheet of glass-filled epoxy, paper, phenolic resin, Kevlar, etc. The sheet is stamped in a stamping die or drilled and cut to form openings for mounting circuit components. A screen then is applied to the sheet. Openings are formed in the screen at strategic locations for forming pads that define electrical connections with the components. The screen acts as a stencil as tin/lead alloy in the form of a cold paste is applied to the screen. The alloy is deposited at the strategic locations on the circuit board.
The printed circuit is comprised in part of copper lines etched on the circuit board by an etching process, thus providing electrical connections between the pads, terminal pins and the circuit components.
The circuit components can be mounted on either side of the board.
It is usual practice to mount circuit components, such as electronic silicon chips, on the circuit board by first forming a plastic package for each circuit component and assembling the packaged component on the board with various fastening techniques. Examples of this assembly procedure may be found in prior art U.S. Pat. Nos. 5,095,404 and 4,978,638. In the device described in the '404 patent, a packaged chip is mounted on a base having a pedestal that extends through an opening formed in a printed circuit board. Mounting pads are formed on one side of the board, and these are connected to the chip by leads in the usual fashion. The mounting plate for the chip is secured to a mounting pad by a thermally conductive epoxy.
In the design of printed circuit boards of this kind, it is necessary to accommodate dissipation of thermal energy from the components, particularly power dies. For example, if the capacity of a power die is about 10 watts, it is not unusual for the junction at the power die to operate at a temperature of approximately 150.degree. C. If, however, the power die operates at high temperatures continuously over a period of time, the reliability of the die would be adversely affected. It has been found that for every 10.degree. C. incremental increase in junction temperature, the reliability of the electronic devices decreases by 50%.
Attempts have been made in prior art constructions to control temperature increase by dissipating thermal energy. In the case of the design of the '404 patent, the mounting plate, which carries a high density integrated circuit chip, is formed with a heat pipe under the printed circuit board. It is attached to a heat spreader which allows the heat produced by the chip to be removed by the heat pipe and distributed to cooling fins. The heat spreader is mounted on the bottom side of the chip. Thus, only a single heat flow path from the chip is established.
In the design shown in the '636 patent, a packaged circuit element is molded into a plastic cover together with a heat sink that forms a part of the package. A thermally conductive interface material is situated between the heat sink portion of the package and one surface of the plastic package.
Prior art U.S. Pat. Nos. 5,168,926 and 5,175,613 are further examples of prior art teachings for forming an assembly of a heat sink with a packaged chip carrier. In each case, a thermally conductive tape or bonding material is situated on one side of the packaged part to form a heat flow path from the part to a heat sink, the latter being secured to the part by bonding material. Heat transfer thus occurs from the part to the sink through a single heat flow path from only one side of the part.
The thermal conductivity of the printed wiring board carrier material is very low. Thus, if a power semiconductor is mounted directly on the board, it is difficult to keep the junction temperature low. It is necessary, therefore, to operate the device at a low power level. If a power semiconductor is mounted directly on the heat sink, as in the case of prior art designs such as those discussed above, only a part of the area of the die is in contact with the heat sink. Furthermore, special assembly equipment and processes are required in the manufacture of power die transistors of this kind. The complexity of the assembly procedure adds substantially to the time and materials required during manufacture.